Silicon carbide (which will be referred to as SiC) has a wide band gap, and its maximum breakdown electric field is larger than that of silicon by one order of magnitude. Thus, SiC has been highly expected to be used as a material for power semiconductor devices in the next generation. As high-quality alpha-phase single crystals, such as 6H-SiC and 4H-SiC, have been increasingly manufactured, various semiconductor devices, such as Shottky diode, MOS field-effect transistor (MOSFET), and thyristor, using SiC as a semiconductor material have been fabricated and tested, and it has been confirmed that these devices exhibit far more excellent characteristics than known devices using silicon.
When SiC is exposed to an oxidizing atmosphere (for example, dry oxygen or water vapor) at a high temperature of 1000.degree. C. to 1200.degree. C., a silicon dioxide film (hereinafter referred to as SiO.sub.2 film) is grown on the surface of SiC, as in the case of silicon. It is also known that the SiO.sub.2 film thus formed provides a desirable interface between an insulating film and a semiconductor substrate. This characteristic is peculiar to SiC, and cannot be observed in other compound semiconductor materials. Since this characteristic may be advantageously utilized to relatively easily produce MOSFET, SiC is expected to be used in a wide range of applications in the future.
Various characteristics have been revealed with respect to growth of SiO.sub.2 film on SiC by thermal oxidation. For example, FIG. 3 is a graph showing the temperature dependency of the growth rate of SiO.sub.2 film on SiC when it was exposed to an atmosphere composed of water vapor in a test conducted by M. R. Melloch and J. A. Copper (MRS Bulletin, March 1997, p.42). For comparison, the graph of FIG. 3 also shows the grown rate of oxide film on silicon. Other reports relating to the growth of Si.sub.2 film on SiC have been published in K. Undo and Y. Seki: "Silicon Carbide and Related Materials 1995" IOP publishing p.629, and A. Golz, G. Horstmann, E. Stein von Kamienski and H. Kurz: "Silicon Carbide and Related Materials 1995" IOP publishing p.633.
As shown in FIG. 3, the growth rate of SiO.sub.2 film on SiC is dependent upon the crystal orientation. Namely, the growth rate in (0001) silicon face (hereinafter referred to as Si-face) is considerably smaller than the growth rate in (000-1) carbon face (hereinafter referred to as C-face). In view of this, it may be considered to use C-face as plane orientation when fabricating SiC semiconductor devices. In fact, the interface state density of the interface between SiO.sub.2 film and SiC when C-face is used for plane orientation is far higher than that in the case where Si face is used for plane orientation, and thus it will be understood that the use of C-face for plane orientation is not appropriate, in particular, in the manufacture of MOS type SiC semiconductor devices. Under these circumstances, Si-face has been generally used in recent development of SiC semiconductor devices.
The small oxidation rate or growth rate as described above means that the SiC substrate needs to be exposed to a high temperature for a long time in order to provide a sufficiently thick oxide film. For example, it is estimated from FIG. 3 that the Si-face of the SiC substrate needs to be exposed to a high temperature of 1100.degree. C. for about 17 hours, in order to provide an oxide film having a thickness of 100 nm.
If the semiconductor substrate is subjected to heat treatment at a high temperature for such a long time, defects may arise in the semiconductor substrate, or other problems may occur during the heat treatment. For this reason, the oxidation time is desired to be reduced to be as short as possible.
Among known methods of thermal oxidation of SiC semiconductor devices, there has been generally employed a so-called wet oxidation method in which pure water is heated so as to cause bubbling of oxygen. In this method, however, it is difficult to control the partial pressure of water vapor, and drops of water are undesirably introduced into the resulting film, which tends to cause a problem of contamination.